From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: [PATCHv2] percpu: make this_cpu_generic_read() atomic w.r.t. interrupts Date: Tue, 26 Sep 2017 07:41:18 -0700 Message-ID: <20170926144118.GA70381@devbig577.frc2.facebook.com> References: <1506426112-19826-1-git-send-email-mark.rutland@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1506426112-19826-1-git-send-email-mark.rutland@arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Mark Rutland Cc: linux-kernel@vger.kernel.org, Arnd Bergmann , Christoph Lameter , Peter Zijlstra , Pranith Kumar , Thomas Gleixner , linux-arch@vger.kernel.org List-Id: linux-arch.vger.kernel.org On Tue, Sep 26, 2017 at 12:41:52PM +0100, Mark Rutland wrote: > As raw_cpu_generic_read() is a plain read from a raw_cpu_ptr() address, > it's possible (albeit unlikely) that the compiler will split the access > across multiple instructions. > > In this_cpu_generic_read() we disable preemption but not interrupts > before calling raw_cpu_generic_read(). Thus, an interrupt could be taken > in the middle of the split load instructions. If a this_cpu_write() or > RMW this_cpu_*() op is made to the same variable in the interrupt > handling path, this_cpu_read() will return a torn value. > > For native word types, we can avoid tearing using READ_ONCE(), but this > won't work in all cases (e.g. 64-bit types on most 32-bit platforms). > This patch reworks this_cpu_generic_read() to use READ_ONCE() where > possible, otherwise falling back to disabling interrupts. > > Signed-off-by: Mark Rutland > Cc: Arnd Bergmann > Cc: Christoph Lameter > Cc: Peter Zijlstra > Cc: Pranith Kumar > Cc: Tejun Heo > Cc: Thomas Gleixner > Cc: linux-arch@vger.kernel.org Applied to percpu/for-4.14-fixes. Thanks. -- tejun