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. But.. > - 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?) -- 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/
