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From: Marco Elver <elver@google.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Florian Weimer <fweimer@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Collingbourne <pcc@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Alexander Potapenko <glider@google.com>,
	sparclinux <sparclinux@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>
Subject: Re: Is perf_sigtrap synchronous?
Date: Sat, 1 May 2021 02:28:33 +0200
Message-ID: <CANpmjNOwUfcCrBfCjtq9ngjqkqjYzehrqS+=+2oA=703tNP=aA@mail.gmail.com> (raw)
In-Reply-To: <m1im43qrug.fsf_-_@fess.ebiederm.org>

On Sat, 1 May 2021 at 01:23, Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> I am looking at perf_sigtrap and I am confused by the code.
>
>
>         /*
>          * We'd expect this to only occur if the irq_work is delayed and either
>          * ctx->task or current has changed in the meantime. This can be the
>          * case on architectures that do not implement arch_irq_work_raise().
>          */
>         if (WARN_ON_ONCE(event->ctx->task != current))
>                 return;
>
>         /*
>          * perf_pending_event() can race with the task exiting.
>          */
>         if (current->flags & PF_EXITING)
>                 return;
>
>
> It performs tests that absolutely can never fail if we are talking about
> a synchronous exception.  The code force_sig family of functions only
> make sense to use with and are only safe to use with synchronous
> exceptions.
>
> Are the tests in perf_sigtrap necessary or is perf_sigtrap not reporting
> a synchronous event?

Yes it's synchronous, insofar that the user will receive the signal
right when the event happens (I've tested this extensively, also see
tools/testing/selftests/perf_events). Of course, there's some effort
involved from the point where the event triggered to actually safely
delivering the signal. In particular, for HW events, these arrive in
NMI, and we can't do much in NMI, and therefore will queue an
irq_work.

On architectures that properly implement irq_work, it will do a
self-IPI, so that once it is safe to do so, another interrupt is
delivered where we process the event and do the force_sig_info(). The
task where the event occurred never got a chance to run -- except for
bad architectures with broken irq_work, and the first WARN_ON() is
there so we don't crash the kernel if somebody botched their irq_work.

Since we're talking about various HW events, these can still trigger
while the task is exiting, before perf_event_exit_task() being called
during do_exit(). That's why we have the 2nd check.

Thanks,
-- Marco

  reply index

Thread overview: 103+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-29  7:48 siginfo_t ABI break on sparc64 from si_addr_lsb move 3y ago Marco Elver
2021-04-29 17:23 ` Eric W. Biederman
2021-04-29 18:46   ` Marco Elver
2021-04-29 20:48   ` Arnd Bergmann
2021-04-30 17:08     ` Eric W. Biederman
2021-04-30 19:07       ` Marco Elver
2021-04-30 20:15         ` Eric W. Biederman
2021-04-30 23:50           ` Marco Elver
2021-04-30 22:49         ` [RFC][PATCH 0/3] signal: Move si_trapno into the _si_fault union Eric W. Biederman
2021-04-30 22:50           ` [PATCH 1/3] siginfo: Move si_trapno inside the union inside _si_fault Eric W. Biederman
2021-05-01 10:31             ` Marco Elver
2021-05-02 18:27               ` Eric W. Biederman
2021-04-30 22:54           ` [PATCH 2/3] signal: Implement SIL_FAULT_TRAPNO Eric W. Biederman
2021-04-30 23:19             ` Eric W. Biederman
2021-05-01 10:33             ` Marco Elver
2021-05-02 18:24               ` Eric W. Biederman
2021-04-30 22:55           ` [PATCH 3/3] signal: Use dedicated helpers to send signals with si_trapno set Eric W. Biederman
2021-05-01 10:33             ` Marco Elver
2021-04-30 22:56           ` [PATCH 4/3] signal: Remove __ARCH_SI_TRAPNO Eric W. Biederman
2021-04-30 23:23           ` Is perf_sigtrap synchronous? Eric W. Biederman
2021-05-01  0:28             ` Marco Elver [this message]
2021-04-30 23:42           ` [PATCH 5/3] signal: Rename SIL_PERF_EVENT SIL_FAULT_PERF_EVENT for consistency Eric W. Biederman
2021-05-01 10:35             ` Marco Elver
2021-04-30 23:43           ` [PATCH 6/3] signal: Factor force_sig_perf out of perf_sigtrap Eric W. Biederman
2021-05-01 10:45             ` Marco Elver
2021-04-30 23:43           ` [PATCH 7/3] signal: Deliver all of the perf_data in si_perf Eric W. Biederman
2021-05-01 10:47             ` Marco Elver
2021-05-02 18:39               ` Eric W. Biederman
2021-05-02 19:13                 ` Marco Elver
2021-05-03 12:44                 ` Peter Zijlstra
2021-05-03 19:38                   ` Eric W. Biederman
2021-05-03 19:53                     ` Marco Elver
2021-04-30 23:47           ` [RFC][PATCH 0/3] signal: Move si_trapno into the _si_fault union Eric W. Biederman
2021-05-01  0:37             ` Marco Elver
2021-05-01 15:16               ` Eric W. Biederman
2021-05-01 16:24                 ` Marco Elver
2021-05-03 20:25                   ` [PATCH 00/12] signal: sort out si_trapno and si_perf Eric W. Biederman
2021-05-03 20:38                     ` [PATCH 01/12] sparc64: Add compile-time asserts for siginfo_t offsets Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 02/12] arm: " Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 03/12] arm64: " Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 04/12] siginfo: Move si_trapno inside the union inside _si_fault Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 05/12] signal: Implement SIL_FAULT_TRAPNO Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 06/12] signal: Use dedicated helpers to send signals with si_trapno set Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 07/12] signal: Remove __ARCH_SI_TRAPNO Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 08/12] signal: Rename SIL_PERF_EVENT SIL_FAULT_PERF_EVENT for consistency Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 09/12] signal: Factor force_sig_perf out of perf_sigtrap Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 10/12] signal: Redefine signinfo so 64bit fields are possible Eric W. Beiderman
2021-05-03 21:04                         ` Eric W. Biederman
2021-05-03 22:47                           ` Marco Elver
2021-05-04  3:42                             ` Eric W. Biederman
2021-05-04  4:03                               ` Peter Collingbourne
2021-05-04  9:52                                 ` Marco Elver
2021-05-04 16:16                                   ` Eric W. Biederman
2021-05-03 20:38                       ` [PATCH 11/12] signal: Deliver all of the siginfo perf data in _perf Eric W. Beiderman
2021-05-03 20:38                       ` [PATCH 12/12] signalfd: Remove SIL_FAULT_PERF_EVENT fields from signalfd_siginfo Eric W. Beiderman
2021-05-04 21:13                     ` [PATCH v3 00/12] signal: sort out si_trapno and si_perf Eric W. Biederman
2021-05-04 22:05                       ` Marco Elver
2021-05-05 14:12                         ` Eric W. Biederman
2021-05-05 14:10                       ` [PATCH v3 01/12] sparc64: Add compile-time asserts for siginfo_t offsets Eric W. Beiderman
2021-05-05 14:10                         ` [PATCH v3 02/12] arm: " Eric W. Beiderman
2021-05-05 14:10                         ` [PATCH v3 03/12] arm64: " Eric W. Beiderman
2021-05-05 14:10                         ` [PATCH v3 04/12] signal: Verify the alignment and size of siginfo_t Eric W. Beiderman
2021-05-05 17:24                           ` Marco Elver
2021-05-05 14:10                         ` [PATCH v3 05/12] siginfo: Move si_trapno inside the union inside _si_fault Eric W. Beiderman
2021-05-05 14:10                         ` [PATCH v3 06/12] signal: Implement SIL_FAULT_TRAPNO Eric W. Beiderman
2021-05-05 17:25                           ` Marco Elver
2021-05-05 14:10                         ` [PATCH v3 07/12] signal: Use dedicated helpers to send signals with si_trapno set Eric W. Beiderman
2021-05-05 17:25                           ` Marco Elver
2021-05-05 14:10                         ` [PATCH v3 08/12] signal: Remove __ARCH_SI_TRAPNO Eric W. Beiderman
2021-05-05 17:25                           ` Marco Elver
2021-05-05 14:10                         ` [PATCH v3 09/12] signal: Rename SIL_PERF_EVENT SIL_FAULT_PERF_EVENT for consistency Eric W. Beiderman
2021-05-05 17:26                           ` Marco Elver
2021-05-05 14:10                         ` [PATCH v3 10/12] signal: Factor force_sig_perf out of perf_sigtrap Eric W. Beiderman
2021-05-05 17:26                           ` Marco Elver
2021-05-06 10:54                           ` Peter Zijlstra
2021-05-05 14:11                         ` [PATCH v3 11/12] signal: Deliver all of the siginfo perf data in _perf Eric W. Beiderman
2021-05-05 17:27                           ` Marco Elver
2021-05-05 14:11                         ` [PATCH v3 12/12] signalfd: Remove SIL_FAULT_PERF_EVENT fields from signalfd_siginfo Eric W. Beiderman
2021-05-05 17:27                           ` Marco Elver
2021-05-05 17:28                       ` [PATCH v3 00/12] signal: sort out si_trapno and si_perf Marco Elver
2021-05-06  7:00                       ` Geert Uytterhoeven
2021-05-06 10:43                         ` Marco Elver
2021-05-06 15:28                           ` Eric W. Biederman
2021-05-06 15:14                         ` Eric W. Biederman
2021-05-14  4:54                       ` [GIT PULL] siginfo: ABI fixes for v5.13-rc2 Eric W. Biederman
2021-05-14 19:14                         ` Linus Torvalds
2021-05-14 21:15                           ` Eric W. Biederman
2021-05-14 22:38                             ` Eric W. Biederman
2021-05-16  7:40                         ` Ingo Molnar
2021-05-17 15:29                           ` Eric W. Biederman
2021-05-21 14:59                         ` [GIT PULL] siginfo: ABI fixes for v5.13-rc3 Eric W. Biederman
2021-05-21 16:34                           ` pr-tracker-bot
2021-05-17 19:56                       ` [PATCH v4 0/5] siginfo: ABI fixes for TRAP_PERF Eric W. Biederman
2021-05-17 19:57                         ` [PATCH v4 1/5] siginfo: Move si_trapno inside the union inside _si_fault Eric W. Beiderman
2021-05-17 19:57                           ` [PATCH v4 2/5] signal: Implement SIL_FAULT_TRAPNO Eric W. Beiderman
2021-05-17 19:57                           ` [PATCH v4 3/5] signal: Factor force_sig_perf out of perf_sigtrap Eric W. Beiderman
2021-05-17 19:57                           ` [PATCH v4 4/5] signal: Deliver all of the siginfo perf data in _perf Eric W. Beiderman
2021-05-17 19:57                           ` [PATCH v4 5/5] signalfd: Remove SIL_PERF_EVENT fields from signalfd_siginfo Eric W. Beiderman
2021-05-17 20:53                         ` [PATCH v4 0/5] siginfo: ABI fixes for TRAP_PERF Marco Elver
2021-05-18  3:46                           ` Eric W. Biederman
2021-05-18  6:44                             ` Marco Elver
2021-05-01 16:26               ` [RFC][PATCH 0/3] signal: Move si_trapno into the _si_fault union Marco Elver
2021-04-30 20:43       ` siginfo_t ABI break on sparc64 from si_addr_lsb move 3y ago Arnd Bergmann

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