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Date: Tue, 10 Aug 2021 20:49:14 +0800 From: Boqun Feng <boqun.feng@...il.com> To: Valentin Schneider <valentin.schneider@....com> Cc: Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, rcu@...r.kernel.org, linux-rt-users@...r.kernel.org, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Steven Rostedt <rostedt@...dmis.org>, Daniel Bristot de Oliveira <bristot@...hat.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, "Paul E. McKenney" <paulmck@...nel.org>, Frederic Weisbecker <frederic@...nel.org>, Josh Triplett <josh@...htriplett.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Davidlohr Bueso <dave@...olabs.net>, Lai Jiangshan <jiangshanlai@...il.com>, Joel Fernandes <joel@...lfernandes.org>, Anshuman Khandual <anshuman.khandual@....com>, Vincenzo Frascino <vincenzo.frascino@....com>, Steven Price <steven.price@....com>, Ard Biesheuvel <ardb@...nel.org> Subject: Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe() On Sun, Aug 08, 2021 at 05:15:20PM +0100, Valentin Schneider wrote: > On 07/08/21 03:42, Mike Galbraith wrote: > > On Sat, 2021-08-07 at 01:58 +0100, Valentin Schneider wrote: > >> > >> +static inline bool is_pcpu_safe(void) > > > > Nit: seems odd to avoid spelling it out to save two characters, percpu > > is word like, rolls off the ole tongue better than p-c-p-u. > > > > -Mike > > True. A quick grep says both versions are used, though "percpu" wins by > about a factor of 2. I'll tweak that for a v3. I wonder why is_percpu_safe() is the correct name. The safety of accesses to percpu variables means two things to me: a) The thread cannot migrate to other CPU in the middle of accessing a percpu variable, in other words, the following cannot happen: { percpu variable X is 0 on CPU 0 and 2 on CPU 1 CPU 0 CPU 1 ======== ========= <in thread A> __this_cpu_inc(X); tmp = X; // tmp is 0 <preempted> <migrate to CPU 1> // continue __this_cpu_inc(X); X = tmp + 1; // CPU 0 miss this // increment (this // may be OK), and // CPU 1's X got // corrupted. b) The accesses to a percpu variable are exclusive, i.e. no interrupt or preemption can happen in the middle of accessing, in other words, the following cannot happen: { percpu variable X is 0 on CPU 0 } CPU 0 ======== <in thread A> __this_cpu_inc(X); tmp = X; // tmp is 0 <preempted> <in other thread> this_cpu_inc(X); // X is 1 afterwards. <back to thread A> X = tmp + 1; // X is 1, and we have a race condition. And the is_p{er}cpu_safe() only detects the first, and it doesn't mean totally safe for percpu accesses. Maybe we can implement a migratable()? Although not sure it's a English word. Regards, Boqun
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