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McKenney" To: Amol Grover Cc: kbuild test robot , kbuild-all@lists.01.org, Corey Minyard , Arnd Bergmann , Greg Kroah-Hartman , openipmi-developer@lists.sourceforge.net, linux-kernel@vger.kernel.org, linux-kernel-mentees@lists.linuxfoundation.org, Joel Fernandes , Madhuparna Bhowmik Subject: Re: [PATCH v2] drivers: char: ipmi: ipmi_msghandler: Pass lockdep expression to RCU lists Message-ID: <20200115193259.GC2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200110164709.26741-1-frextrite@gmail.com> <202001121358.YVbD4V9l%lkp@intel.com> <20200114030030.GB2559@workstation-portable> <20200114175828.GR2935@paulmck-ThinkPad-P72> <20200115123653.GA20601@workstation-portable> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200115123653.GA20601@workstation-portable> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 15, 2020 at 06:06:53PM +0530, Amol Grover wrote: > On Tue, Jan 14, 2020 at 09:58:28AM -0800, Paul E. McKenney wrote: > > On Tue, Jan 14, 2020 at 08:30:30AM +0530, Amol Grover wrote: > > > On Sun, Jan 12, 2020 at 01:25:58PM +0800, kbuild test robot wrote: > > > > Hi Amol, > > > > > > > > Thank you for the patch! Perhaps something to improve: > > > > > > > > [auto build test WARNING on char-misc/char-misc-testing] > > > > [also build test WARNING on ipmi/for-next arm-soc/for-next v5.5-rc5] > > > > [if your patch is applied to the wrong git tree, please drop us a note to help > > > > improve the system. BTW, we also suggest to use '--base' option to specify the > > > > base tree in git format-patch, please see https://stackoverflow.com/a/37406982] > > > > > > > > url: https://github.com/0day-ci/linux/commits/Amol-Grover/drivers-char-ipmi-ipmi_msghandler-Pass-lockdep-expression-to-RCU-lists/20200111-081002 > > > > base: https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git 16bb7abc4a6b9defffa294e4dc28383e62a1dbcf > > > > config: x86_64-randconfig-a003-20200109 (attached as .config) > > > > compiler: gcc-5 (Ubuntu 5.5.0-12ubuntu1) 5.5.0 20171010 > > > > reproduce: > > > > # save the attached .config to linux build tree > > > > make ARCH=x86_64 > > > > > > > > If you fix the issue, kindly add following tag > > > > Reported-by: kbuild test robot > > > > > > > > All warnings (new ones prefixed by >>): > > > > > > > > In file included from include/linux/export.h:43:0, > > > > from include/linux/linkage.h:7, > > > > from include/linux/kernel.h:8, > > > > from include/linux/list.h:9, > > > > from include/linux/module.h:12, > > > > from drivers/char/ipmi/ipmi_msghandler.c:17: > > > > drivers/char/ipmi/ipmi_msghandler.c: In function 'find_cmd_rcvr': > > > > include/linux/rculist.h:53:25: warning: suggest parentheses around '&&' within '||' [-Wparentheses] > > > > RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(), \ > > > > ^ > > > > > > As mentioned above, RCU_LOCKDEP_WARN macro is called from > > > __list_check_rcu with 2 parameters > > > > > > 1. !cond && !rcu_read_lock_any_held() > > > 2. The message to display incase there is a lockdep warning. > > > > > > > > > However, if I pass the lockdep checking condition as: > > > > > > list_for_each_entry_rcu(ptr, list, head, lockdep_is_held(&some_lock) || rcu_read_lock_held()) > > > > Right, given the _rcu() suffix on the command, the rcu_read_lock_held() > > is implied. > > > > > this trickles down to __list_check_rcu and then finally to > > > RCU_LOCKDEP_WARN as (here cond is `lockdep_is_held(&some_lock) || rcu_read_lock_held()`): > > > > > > RCU_LOCKDEP_WARN(!lockdep_is_held(&some_lock) || rcu_read_lock_held() && !rcu_read_lock_any_held()) > > > > > > which according to operator precedence (I hopefully got them right) > > > would always evaluate to true if we are in an RCU read-side critical > > > section (without a lock), and hence, result in a false-positive lockdep > > > warning. > > > > It looks that way to me. But why not actually try it out? After all, > > only the running system knows for sure. And there might be some trick > > that we are both missing. > > > > I just tested this, here are the results: > > Case 1: Using`lockdep_is_held() || rcu_read_lock_held()` > > lock RCU RSCS Splat? Actual > Y Y N N > Y N N N > N Y Y N <= > N N Y Y > > Similar for > Case 2: Using `rcu_read_lock_held() || lockdep_is_held()` > > Case 3: Consider 2 locks (outside rcu_read_lock()) > `lockdep_is_held(lock1) || lockdep_is_held(lock2)` > > lock1 lock2 Splat? Actual > Y Y N* N > Y N N N > N Y Y N <= > N N Y Y > > This too proves the hypothesis (I'd like to call that). Very good! > *However, this shows an interesting result. When both lock1 and lock2 > are held, according to the hypothesis, a splat should've occured, since > the check condition (albeit faulty atm) would be: > > `!lockdep_is_held(lock1) || lockdep_is_held(lock2) && !rcu_read_lock_any_held()` > => `!T || T && !F` > => `F || T && T` > => `F || T` > => `T` > However, there was no splat. Which led me to investigate further and I > found out: > 1. `rcu_read_lock_any_held()` always returns 1 even if it is outside RCU > read-side CS. > 2. `rcu_read_lock_held()` seems OK, returns 1 when inside and 0 when > outside > > The kernel is compiled with > PROVE_RCU=y > PROVE_RCU_LIST=y Were you within a preempt-disable region, for example, was some other spinlock held? (A lockdep splat should give you a list of locks held.) Both of these act as generalized RCU read-side critical sections in recent kernels. > Any thoughts on this? Is this intended? And should I send-in the patch > for the first problem? Separate patches for the initial problem and fixing the macro argument, please, if that is what you are asking. Thanx, Paul > Thanks > Amol > > > > This could be easily solved by putting `cond` inside brackets as it is > > > correctly done in RCU_LOCKDEP_WARN macro but not in __list_check_rcu > > > macro. Is that so, or did I miss something? > > > > Again, that looks correct to me, but please check. > > > > > Secondly, since there is already a condition that checks for RCU > > > read-side critical section, the extra `rcu_read_lock_held()` we supply > > > is sort of redundant and can be skipped right? > > > > Yes, the general rule is that if the primitives ends with _rcu(), any > > lockdep condition will be in addition to rcu_read_lock_any_held(). > > So you should not need to pass RCU read-side lockdep expressions to > > primitives whose names end in _rcu.. > > > > Thanx, Paul > > > > > Thanks > > > Amol > > > > > > > include/linux/compiler.h:58:52: note: in definition of macro '__trace_if_var' > > > > #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond)) > > > > ^ > > > > >> include/linux/rcupdate.h:263:3: note: in expansion of macro 'if' > > > > if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \ > > > > ^ > > > > >> include/linux/rculist.h:53:2: note: in expansion of macro 'RCU_LOCKDEP_WARN' > > > > RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(), \ > > > > ^ > > > > include/linux/rculist.h:371:7: note: in expansion of macro '__list_check_rcu' > > > > for (__list_check_rcu(dummy, ## cond, 0), \ > > > > ^ > > > > drivers/char/ipmi/ipmi_msghandler.c:1607:2: note: in expansion of macro 'list_for_each_entry_rcu' > > > > list_for_each_entry_rcu(rcvr, &intf->cmd_rcvrs, link, > > > > ^ > > > > include/linux/rculist.h:53:25: warning: suggest parentheses around '&&' within '||' [-Wparentheses] > > > > RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(), \ > > > > ^ > > > > include/linux/compiler.h:58:61: note: in definition of macro '__trace_if_var' > > > > #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond)) > > > > ^ > > > > >> include/linux/rcupdate.h:263:3: note: in expansion of macro 'if' > > > > if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \ > > > > ^ > > > > >> include/linux/rculist.h:53:2: note: in expansion of macro 'RCU_LOCKDEP_WARN' > > > > RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(), \ > > > > ^ > > > > include/linux/rculist.h:371:7: note: in expansion of macro '__list_check_rcu' > > > > for (__list_check_rcu(dummy, ## cond, 0), \ > > > > ^ > > > > drivers/char/ipmi/ipmi_msghandler.c:1607:2: note: in expansion of macro 'list_for_each_entry_rcu' > > > > list_for_each_entry_rcu(rcvr, &intf->cmd_rcvrs, link, > > > > ^ > > > > include/linux/rculist.h:53:25: warning: suggest parentheses around '&&' within '||' [-Wparentheses] > > > > RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(), \ > > > > ^ > > > > include/linux/compiler.h:69:3: note: in definition of macro '__trace_if_value' > > > > (cond) ? \ > > > > ^ > > > > include/linux/compiler.h:56:28: note: in expansion of macro '__trace_if_var' > > > > #define if(cond, ...) if ( __trace_if_var( !!(cond , ## __VA_ARGS__) ) ) > > > > ^ > > > > >> include/linux/rcupdate.h:263:3: note: in expansion of macro 'if' > > > > if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \ > > > > ^ > > > > >> include/linux/rculist.h:53:2: note: in expansion of macro 'RCU_LOCKDEP_WARN' > > > > RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(), \ > > > > ^ > > > > include/linux/rculist.h:371:7: note: in expansion of macro '__list_check_rcu' > > > > for (__list_check_rcu(dummy, ## cond, 0), \ > > > > ^ > > > > drivers/char/ipmi/ipmi_msghandler.c:1607:2: note: in expansion of macro 'list_for_each_entry_rcu' > > > > list_for_each_entry_rcu(rcvr, &intf->cmd_rcvrs, link, > > > > ^ > > > > > > > > vim +/if +263 include/linux/rcupdate.h > > > > > > > > 632ee200130899 Paul E. McKenney 2010-02-22 254 > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 255 /** > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 256 * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 257 * @c: condition to check > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 258 * @s: informative message > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 259 */ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 260 #define RCU_LOCKDEP_WARN(c, s) \ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 261 do { \ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 262 static bool __section(.data.unlikely) __warned; \ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 @263 if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 264 __warned = true; \ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 265 lockdep_rcu_suspicious(__FILE__, __LINE__, s); \ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 266 } \ > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 267 } while (0) > > > > f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 268 > > > > > > > > :::::: The code at line 263 was first introduced by commit > > > > :::::: f78f5b90c4ffa559e400c3919a02236101f29f3f rcu: Rename rcu_lockdep_assert() to RCU_LOCKDEP_WARN() > > > > > > > > :::::: TO: Paul E. McKenney > > > > :::::: CC: Paul E. McKenney > > > > > > > > --- > > > > 0-DAY kernel test infrastructure Open Source Technology Center > > > > https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org Intel Corporation > > > > > >