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During the virtual engine's submission tasklet, we take the request and insert into the submission queue on each of our siblings. This seems quite simply, and so no problems with ordering. However, the sibling execlists' submission tasklets may run concurrently with the virtual engine's tasklet, submitting the request to HW before the virtual finishes its task of telling all the siblings. If this happens, the sibling tasklet may *reorder* the ve->sibling[] array that the virtual engine tasklet is processing. This can *only* reorder within the elements already processed by the virtual engine, nevertheless the race is detected by KCSAN: [ 185.580014] BUG: KCSAN: data-race in execlists_dequeue [i915] / virtual_submission_tasklet [i915] [ 185.580054] [ 185.580076] write to 0xffff8881f1919860 of 8 bytes by interrupt on cpu 2: [ 185.580553] execlists_dequeue+0x6ad/0x1600 [i915] [ 185.581044] __execlists_submission_tasklet+0x48/0x60 [i915] [ 185.581517] execlists_submission_tasklet+0xd3/0x170 [i915] [ 185.581554] tasklet_action_common.isra.0+0x42/0x90 [ 185.581585] __do_softirq+0xc8/0x206 [ 185.581613] run_ksoftirqd+0x15/0x20 [ 185.581641] smpboot_thread_fn+0x15a/0x270 [ 185.581669] kthread+0x19a/0x1e0 [ 185.581695] ret_from_fork+0x1f/0x30 [ 185.581717] [ 185.581736] read to 0xffff8881f1919860 of 8 bytes by interrupt on cpu 0: [ 185.582231] virtual_submission_tasklet+0x10e/0x5c0 [i915] [ 185.582265] tasklet_action_common.isra.0+0x42/0x90 [ 185.582291] __do_softirq+0xc8/0x206 [ 185.582315] run_ksoftirqd+0x15/0x20 [ 185.582340] smpboot_thread_fn+0x15a/0x270 [ 185.582368] kthread+0x19a/0x1e0 [ 185.582395] ret_from_fork+0x1f/0x30 [ 185.582417] We can prevent this race by checking for the ve->request after looking up the sibling array. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20200423115315.26825-1-chris@chris-wilson.co.uk
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.