timers, sched/clock: Match scope of read and write seqcounts (8710e914) · Commits · 戴 / test

Currently the scope of the raw_write_seqcount_begin/end() in sched_clock_register() far exceeds the scope of the read section in sched_clock(). This gives the impression of safety during cursory review but achieves little. Note that this is likely to be a latent issue at present because sched_clock_register() is typically called before we enable interrupts, however the issue does risk bugs being needlessly introduced as the code evolves. This patch fixes the problem by increasing the scope of the read locking performed by sched_clock() to cover all data modified by sched_clock_register. We also improve clarity by moving writes to struct clock_data that do not impact sched_clock() outside of the critical section. Signed-off-by:

Daniel Thompson <daniel.thompson@linaro.org> [ Reworked it slightly to apply to tip/timers/core] Signed-off-by:

John Stultz <john.stultz@linaro.org> Reviewed-by:

Stephen Boyd <sboyd@codeaurora.org> Acked-by:

Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Link: http://lkml.kernel.org/r/1427397806-20889-2-git-send-email-john.stultz@linaro.org Signed-off-by:

Ingo Molnar <mingo@kernel.org>

kernel/time/sched_clock.c

+11 −15

Original line number	Diff line number	Diff line
		@@ -58,23 +58,21 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)

		unsigned long long notrace sched_clock(void)
		{
		u64 epoch_ns;
		u64 epoch_cyc;
		u64 cyc;
		u64 cyc, res;
		unsigned long seq;

		if (cd.suspended)
		return cd.epoch_ns;

		do {
		seq = raw_read_seqcount_begin(&cd.seq);
		epoch_cyc = cd.epoch_cyc;
		epoch_ns = cd.epoch_ns;
		} while (read_seqcount_retry(&cd.seq, seq));

		res = cd.epoch_ns;
		if (!cd.suspended) {
		cyc = read_sched_clock();
		cyc = (cyc - epoch_cyc) & sched_clock_mask;
		return epoch_ns + cyc_to_ns(cyc, cd.mult, cd.shift);
		cyc = (cyc - cd.epoch_cyc) & sched_clock_mask;
		res += cyc_to_ns(cyc, cd.mult, cd.shift);
		}
		} while (read_seqcount_retry(&cd.seq, seq));

		return res;
		}

		/*
		@@ -111,7 +109,6 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
		{
		u64 res, wrap, new_mask, new_epoch, cyc, ns;
		u32 new_mult, new_shift;
		ktime_t new_wrap_kt;
		unsigned long r;
		char r_unit;

		@@ -124,10 +121,11 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
		clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);

		new_mask = CLOCKSOURCE_MASK(bits);
		cd.rate = rate;

		/* calculate how many nanosecs until we risk wrapping */
		wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask, NULL);
		new_wrap_kt = ns_to_ktime(wrap);
		cd.wrap_kt = ns_to_ktime(wrap);

		/* update epoch for new counter and update epoch_ns from old counter*/
		new_epoch = read();
		@@ -138,8 +136,6 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
		raw_write_seqcount_begin(&cd.seq);
		read_sched_clock = read;
		sched_clock_mask = new_mask;
		cd.rate = rate;
		cd.wrap_kt = new_wrap_kt;
		cd.mult = new_mult;
		cd.shift = new_shift;
		cd.epoch_cyc = new_epoch;

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