On Thu, Jan 26, 2023 at 11:45:37AM +0100, Geert Uytterhoeven wrote: > It is considered good practice to call cpu_relax() in busy loops, see > Documentation/process/volatile-considered-harmful.rst. This can not > only lower CPU power consumption or yield to a hyperthreaded twin > processor, but also allows an architecture to mitigate hardware issues > (e.g. ARM Erratum 754327 for Cortex-A9 prior to r2p0) in the > architecture-specific cpu_relax() implementation. > > As the iopoll helpers lack calls to cpu_relax(), people are sometimes > reluctant to use them, and may fall back to open-coded polling loops > (including cpu_relax() calls) instead. > > Fix this by adding calls to cpu_relax() to the iopoll helpers: > - For the non-atomic case, it is sufficient to call cpu_relax() in > case of a zero sleep-between-reads value, as a call to > usleep_range() is a safe barrier otherwise. > - For the atomic case, cpu_relax() must be called regardless of the > sleep-between-reads value, as there is no guarantee all > architecture-specific implementations of udelay() handle this. > > Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx> In addition to these dodgy architecture fails, cpu_relax() is also a compiler barrier, it is not immediately obvious that the @op argument 'function' will result in an actual function call (inlining ftw). Where a function call is a C sequence point, this is lost on inlining. Therefore, with agressive enough optimization it might be possible for the compiler to hoist the: (val) = op(args); 'load' out of the loop because it doesn't see the value changing. The addition of cpu_relax() will inhibit this. Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > --- > Resent with a larger audience due to lack of comments. > > This has been discussed before, but I am not aware of any patches moving > forward: > - "Re: [PATCH 6/7] clk: renesas: rcar-gen3: Add custom clock for PLLs" > https://lore.kernel.org/all/CAMuHMdWUEhs=nwP+a0vO2jOzkq-7FEOqcJ+SsxAGNXX1PQ2KMA@xxxxxxxxxxxxxx/ > - "Re: [PATCH v2] clk: samsung: Prevent potential endless loop in the PLL set_rate ops" > https://lore.kernel.org/all/20200811164628.GA7958@kozik-lap > --- > include/linux/iopoll.h | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/include/linux/iopoll.h b/include/linux/iopoll.h > index 2c8860e406bd8cae..73132721d1891a2e 100644 > --- a/include/linux/iopoll.h > +++ b/include/linux/iopoll.h > @@ -53,6 +53,8 @@ > } \ > if (__sleep_us) \ > usleep_range((__sleep_us >> 2) + 1, __sleep_us); \ > + else \ > + cpu_relax(); \ There's a simplicitly argument to be had for making it unconditional here too I suppose. usleep() is 'slow' anyway. > } \ > (cond) ? 0 : -ETIMEDOUT; \ > }) > @@ -95,6 +97,7 @@ > } \ > if (__delay_us) \ > udelay(__delay_us); \ > + cpu_relax(); \ > } \ > (cond) ? 0 : -ETIMEDOUT; \ > }) > -- > 2.34.1 >
