On 04/25/2015 05:43 AM, Heiko Carstens wrote: > On Fri, Apr 24, 2015 at 11:16:53AM -0400, Rik van Riel wrote: >> V2: introduce signed_cputime_t to deal with 64 bit cputime_t on >> 32 bit architectures, and use READ_ONCE to ensure the value >> is always read atomically (Heiko Karstens) > > Erm, that's not what I said ;) > READ_ONCE() only fixes the isssue that with your previous code the > compiler was free to generate code that accesses the memory value > several times.
Ah indeed, you are right. >> - local_irq_save(flags); >> time = stime + utime; >> - dtime = time - tsk->acct_timexpd; >> + dtime = time - READ_ONCE(tsk->acct_timexpd); >> + /* >> + * This code is called both from irq context and from >> + * task context. There is a race where irq context advances >> + * tsk->acct_timexpd to a value larger than time, creating >> + * a negative value. In that case, the irq has already >> + * updated the statistics. >> + */ >> + if (unlikely((signed_cputime_t)dtime <= 0)) >> + return; >> + > > ...the READ_ONCE() doesn't give you any guarantees about reading > tsk->acct_timexpd in an atomic way. > Well, actually you don't need atomic semantics, but only to make sure that > the read access happens with a single instruction, since you want to protect > against interrupts. > But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still result > in two instructions on 32 bit architectures. > (or isn't there currently no 32 bit architecture with 64 bit cputime_t left?) Even if there is (maybe some ARM system?), can we even guarantee that a single instruction to read 64 bits exists on such a system? -- All rights reversed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
