dbfb089d36
The recent commit:c6e7bd7afa("sched/core: Optimize ttwu() spinning on p->on_cpu") moved these lines in ttwu(): p->sched_contributes_to_load = !!task_contributes_to_load(p); p->state = TASK_WAKING; up before: smp_cond_load_acquire(&p->on_cpu, !VAL); into the 'p->on_rq == 0' block, with the thinking that once we hit schedule() the current task cannot change it's ->state anymore. And while this is true, it is both incorrect and flawed. It is incorrect in that we need at least an ACQUIRE on 'p->on_rq == 0' to avoid weak hardware from re-ordering things for us. This can fairly easily be achieved by relying on the control-dependency already in place. The second problem, which makes the flaw in the original argument, is that while schedule() will not change prev->state, it will read it a number of times (arguably too many times since it's marked volatile). The previous condition 'p->on_cpu == 0' was sufficient because that indicates schedule() has completed, and will no longer read prev->state. So now the trick is to make this same true for the (much) earlier 'prev->on_rq == 0' case. Furthermore, in order to make the ordering stick, the 'prev->on_rq = 0' assignment needs to he a RELEASE, but adding additional ordering to schedule() is an unwelcome proposition at the best of times, doubly so for mere accounting. Luckily we can push the prev->state load up before rq->lock, with the only caveat that we then have to re-read the state after. However, we know that if it changed, we no longer have to worry about the blocking path. This gives us the required ordering, if we block, we did the prev->state load before an (effective) smp_mb() and the p->on_rq store needs not change. With this we end up with the effective ordering: LOAD p->state LOAD-ACQUIRE p->on_rq == 0 MB STORE p->on_rq, 0 STORE p->state, TASK_WAKING which ensures the TASK_WAKING store happens after the prev->state load, and all is well again. Fixes:c6e7bd7afa("sched/core: Optimize ttwu() spinning on p->on_cpu") Reported-by: Dave Jones <davej@codemonkey.org.uk> Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Dave Jones <davej@codemonkey.org.uk> Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com> Link: https://lkml.kernel.org/r/20200707102957.GN117543@hirez.programming.kicks-ass.net
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.