From: Michael Ellerman <mpe@ellerman.id.au>
To: Timothy Pearson <tpearson@raptorengineering.com>
Cc: Jens Axboe <axboe@kernel.dk>,
regressions <regressions@lists.linux.dev>,
npiggin <npiggin@gmail.com>,
christophe leroy <christophe.leroy@csgroup.eu>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v2] powerpc: Don't clobber fr0/vs0 during fp|altivec register save
Date: Wed, 22 Nov 2023 16:01:50 +1100 [thread overview]
Message-ID: <87leaqjs8x.fsf@mail.lhotse> (raw)
In-Reply-To: <1294229534.48922001.1700539832331.JavaMail.zimbra@raptorengineeringinc.com>
Timothy Pearson <tpearson@raptorengineering.com> writes:
>
...
>
> So a little more detail on this, just to put it to rest properly vs.
> assuming hand analysis caught every possible pathway. :)
>
> The debugging that generates this stack trace also verifies the following in __giveup_fpu():
>
> 1.) tsk->thread.fp_state.fpr doesn't contain the FPSCR contents prior to calling save_fpu()
> 2.) tsk->thread.fp_state.fpr contains the FPSCR contents directly after calling save_fpu()
> 3.) MSR_FP is set both in the task struct and in the live MSR.
>
> Only if all three conditions are met will it generate the trace. This
> is a generalization of the hack I used to find the problem in the
> first place.
>
> If the state will subsequently be reloaded from the thread struct,
> that means we're reloading the registers from the thread struct that
> we just verified was corrupted by the earlier save_fpu() call. There
> are only two ways I can see for that to be true -- one is if the
> registers were already clobbered when giveup_all() was entered, and
> the other is if save_fpu() went ahead and clobbered them right here
> inside giveup_all().
>
> To see which scenario we were dealing with, I added a bit more
> instrumentation to dump the current register state if MSR_FP bit was
> already set in registers (i.e. not dumping data from task struct, but
> using the live FPU registers instead), and sure enough the registers
> are corrupt on entry, so something else has already called save_fpu()
> before we even hit giveup_all() in this call chain.
Can you share the debug patch you're using?
cheers
WARNING: multiple messages have this Message-ID (diff)
From: Michael Ellerman <mpe@ellerman.id.au>
To: Timothy Pearson <tpearson@raptorengineering.com>
Cc: Jens Axboe <axboe@kernel.dk>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
regressions <regressions@lists.linux.dev>,
npiggin <npiggin@gmail.com>
Subject: Re: [PATCH v2] powerpc: Don't clobber fr0/vs0 during fp|altivec register save
Date: Wed, 22 Nov 2023 16:01:50 +1100 [thread overview]
Message-ID: <87leaqjs8x.fsf@mail.lhotse> (raw)
In-Reply-To: <1294229534.48922001.1700539832331.JavaMail.zimbra@raptorengineeringinc.com>
Timothy Pearson <tpearson@raptorengineering.com> writes:
>
...
>
> So a little more detail on this, just to put it to rest properly vs.
> assuming hand analysis caught every possible pathway. :)
>
> The debugging that generates this stack trace also verifies the following in __giveup_fpu():
>
> 1.) tsk->thread.fp_state.fpr doesn't contain the FPSCR contents prior to calling save_fpu()
> 2.) tsk->thread.fp_state.fpr contains the FPSCR contents directly after calling save_fpu()
> 3.) MSR_FP is set both in the task struct and in the live MSR.
>
> Only if all three conditions are met will it generate the trace. This
> is a generalization of the hack I used to find the problem in the
> first place.
>
> If the state will subsequently be reloaded from the thread struct,
> that means we're reloading the registers from the thread struct that
> we just verified was corrupted by the earlier save_fpu() call. There
> are only two ways I can see for that to be true -- one is if the
> registers were already clobbered when giveup_all() was entered, and
> the other is if save_fpu() went ahead and clobbered them right here
> inside giveup_all().
>
> To see which scenario we were dealing with, I added a bit more
> instrumentation to dump the current register state if MSR_FP bit was
> already set in registers (i.e. not dumping data from task struct, but
> using the live FPU registers instead), and sure enough the registers
> are corrupt on entry, so something else has already called save_fpu()
> before we even hit giveup_all() in this call chain.
Can you share the debug patch you're using?
cheers
next prev parent reply other threads:[~2023-11-22 5:02 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-19 15:18 [PATCH v2] powerpc: Don't clobber fr0/vs0 during fp|altivec register save Timothy Pearson
2023-11-20 7:10 ` Michael Ellerman
2023-11-20 14:32 ` Nicholas Piggin
2023-11-20 16:45 ` Timothy Pearson
2023-11-20 16:45 ` Timothy Pearson
2023-11-20 23:39 ` Michael Ellerman
2023-11-20 23:39 ` Michael Ellerman
2023-11-21 0:27 ` Nicholas Piggin
2023-11-21 0:27 ` Nicholas Piggin
2023-11-21 1:23 ` Timothy Pearson
2023-11-21 1:23 ` Timothy Pearson
2023-11-21 7:56 ` Nicholas Piggin
2023-11-21 7:56 ` Nicholas Piggin
2023-11-21 4:10 ` Timothy Pearson
2023-11-21 4:10 ` Timothy Pearson
2023-11-21 4:26 ` Timothy Pearson
2023-11-21 4:26 ` Timothy Pearson
2023-11-21 7:54 ` Nicholas Piggin
2023-11-21 7:54 ` Nicholas Piggin
2023-11-22 5:01 ` Michael Ellerman [this message]
2023-11-22 5:01 ` Michael Ellerman
2023-11-24 0:01 ` Timothy Pearson
2023-11-24 0:01 ` Timothy Pearson
2023-11-27 18:39 ` Timothy Pearson
2023-11-27 18:39 ` Timothy Pearson
2023-11-27 19:58 ` Christophe Leroy
2023-11-27 19:58 ` Christophe Leroy
2023-11-28 0:59 ` Michael Ellerman
2023-11-28 0:59 ` Michael Ellerman
2023-11-28 1:40 ` Nicholas Piggin
2023-11-28 1:40 ` Nicholas Piggin
2023-11-27 22:53 ` Michael Ellerman
2023-11-27 22:53 ` Michael Ellerman
2023-11-28 12:57 ` Michael Ellerman
2023-11-28 12:57 ` Michael Ellerman
2023-11-30 16:29 ` Timothy Pearson
2023-11-30 16:29 ` Timothy Pearson
2023-11-21 0:18 ` Nicholas Piggin
2023-11-21 0:18 ` Nicholas Piggin
2023-12-02 23:00 ` Michael Ellerman
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