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Date:   Wed, 29 Aug 2018 13:13:10 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Nadav Amit <namit@vmware.com>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, X86 ML <x86@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        linux-arch <linux-arch@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [RFC PATCH 1/6] x86/alternative: assert text_mutex is taken
Message-ID: <20180829201309.GA7142@linux.intel.com>
References: <20180829081147.184610-1-namit@vmware.com>
 <20180829081147.184610-2-namit@vmware.com>
 <20180829175936.fb27b3bf13da819a9a971f07@kernel.org>
 <1F547CEE-B5D9-42A0-8093-2C5555BACE26@vmware.com>
 <2694AE6F-2212-46C6-A570-6BAF265364FB@vmware.com>
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On Wed, Aug 29, 2018 at 07:36:22PM +0000, Nadav Amit wrote:
> at 10:11 AM, Nadav Amit <namit@vmware.com> wrote:
> 
> > at 1:59 AM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > 
> >> On Wed, 29 Aug 2018 01:11:42 -0700
> >> Nadav Amit <namit@vmware.com> wrote:
> >> 
> >>> Use lockdep to ensure that text_mutex is taken when text_poke() is
> >>> called.
> >>> 
> >>> Actually it is not always taken, specifically when it is called by kgdb,
> >>> so take the lock in these cases.
> >> 
> >> Can we really take a mutex in kgdb context?
> >> 
> >> kgdb_arch_remove_breakpoint
> >> <- dbg_deactivate_sw_breakpoints
> >>   <- kgdb_reenter_check
> >>      <- kgdb_handle_exception
> >>         <- __kgdb_notify
> >>           <- kgdb_ll_trap
> >>             <- do_int3
> >>           <- kgdb_notify
> >>             <- die notifier
> >> 
> >> kgdb_arch_set_breakpoint
> >> <- dbg_activate_sw_breakpoints
> >>   <- kgdb_reenter_check
> >>      <- kgdb_handle_exception
> >>          ...
> >> 
> >> Both seems called in exception context, so we can not take a mutex lock.
> >> I think kgdb needs a special path.
> > 
> > You are correct, but I don’t want a special path. Presumably text_mutex is
> > guaranteed not to be taken according to the code.
> > 
> > So I guess the only concern is lockdep. Do you see any problem if I change
> > mutex_lock() into mutex_trylock()? It should always succeed, and I can add a
> > warning and a failure path if it fails for some reason.
> 
> Err.. This will not work. I think I will drop this patch, since I cannot
> find a proper yet simple assertion. Creating special path just for the
> assertion seems wrong.

It's probably worth expanding the comment for text_poke() to call out
the kgdb case and reference kgdb_arch_{set,remove}_breakpoint(), whose
code and comments make it explicitly clear why its safe for them to
call text_poke() without acquiring the lock.  Might prevent someone
from going down this path again in the future.