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Date:   Tue, 14 Jan 2020 08:30:30 +0530
From:   Amol Grover <frextrite@gmail.com>
To:     kbuild test robot <lkp@intel.com>
Cc:     kbuild-all@lists.01.org, Corey Minyard <minyard@acm.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        openipmi-developer@lists.sourceforge.net,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Joel Fernandes <joel@joelfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH v2] drivers: char: ipmi: ipmi_msghandler: Pass lockdep
 expression to RCU lists
Message-ID: <20200114030030.GB2559@workstation-portable>
References: <20200110164709.26741-1-frextrite@gmail.com>
 <202001121358.YVbD4V9l%lkp@intel.com>
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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 <lkp@intel.com>
> 
> 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())

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.

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?

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?

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 <paulmck@linux.vnet.ibm.com>
> :::::: CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> ---
> 0-DAY kernel test infrastructure                 Open Source Technology Center
> https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org Intel Corporation