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X-OriginatorOrg: axis.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Oct 2023 14:47:43.2988 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: b48aaed4-dcfe-49a5-3103-08dbd17b8444 X-MS-Exchange-CrossTenant-Id: 78703d3c-b907-432f-b066-88f7af9ca3af X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=78703d3c-b907-432f-b066-88f7af9ca3af;Ip=[195.60.68.100];Helo=[mail.axis.com] X-MS-Exchange-CrossTenant-AuthSource: AM1PEPF000252DC.eurprd07.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS2PR02MB9692 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In basic time travel mode, I sometimes see "time goes backwards" panics like the one below: Kernel panic: time-travel: time goes backwards 161689340000492 -> 16168933= 9869814 Call Trace: panic+0x1a1/0x3d7 time_travel_update_time.cold+0xe9/0x133 timer_read+0xc1/0x100 ktime_get+0x10c/0x200 copy_process+0x1899/0x2230 kernel_clone+0x57/0x7a0 kernel_thread+0x4a/0x50 kthreadd+0x116/0x190 The problem is a race between time_travel_handle_real_alarm() and timer_read(). time_travel_handle_real_alarm() changes the time after time_read() reads the current time but before time_travel_update_time() has had a chance to add the end event. Fix this by doing the time read and event add atomically with respect to time_travel_handle_real_alarm(). Signed-off-by: Vincent Whitchurch --- arch/um/kernel/time.c | 48 ++++++++++++++++++++++++++++++++++-------------- 1 file changed, 34 insertions(+), 14 deletions(-) diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c index fddd1dec27e6..38a94dd41b8f 100644 --- a/arch/um/kernel/time.c +++ b/arch/um/kernel/time.c @@ -392,17 +392,11 @@ bool time_travel_del_event(struct time_travel_event *= e) return true; } =20 -static void time_travel_update_time(unsigned long long next, bool idle) +static void time_travel_update_to_event(struct time_travel_event *ne, bool= idle) { - struct time_travel_event ne =3D { - .onstack =3D true, - }; struct time_travel_event *e; bool finished =3D idle; =20 - /* add it without a handler - we deal with that specifically below */ - __time_travel_add_event(&ne, next); - do { e =3D time_travel_first_event(); =20 @@ -414,7 +408,7 @@ static void time_travel_update_time(unsigned long long = next, bool idle) BUG_ON(!time_travel_del_event(e)); BUG_ON(time_travel_time !=3D e->time); =20 - if (e =3D=3D &ne) { + if (e =3D=3D ne) { finished =3D true; } else { if (e->onstack) @@ -427,14 +421,38 @@ static void time_travel_update_time(unsigned long lon= g next, bool idle) e =3D time_travel_first_event(); if (e) time_travel_ext_update_request(e->time); - } while (ne.pending && !finished); + } while (ne->pending && !finished); + + time_travel_del_event(ne); +} =20 - time_travel_del_event(&ne); +static void time_travel_update_time(unsigned long long next, bool idle) +{ + struct time_travel_event ne =3D { + .onstack =3D true, + }; + + __time_travel_add_event(&ne, next); + time_travel_update_to_event(&ne, idle); +} + +static void time_travel_forward_time(unsigned long nsec, bool idle) +{ + struct time_travel_event ne =3D { + .onstack =3D true, + }; + unsigned long flags; + + local_irq_save(flags); + __time_travel_add_event(&ne, time_travel_time + nsec); + local_irq_restore(flags); + + time_travel_update_to_event(&ne, idle); } =20 void time_travel_ndelay(unsigned long nsec) { - time_travel_update_time(time_travel_time + nsec, false); + time_travel_forward_time(nsec, false); } EXPORT_SYMBOL(time_travel_ndelay); =20 @@ -572,6 +590,10 @@ static inline void time_travel_update_time(unsigned lo= ng long ns, bool retearly) { } =20 +static void time_travel_forward_time(unsigned long nsec, bool idle) +{ +} + static inline void time_travel_handle_real_alarm(void) { } @@ -720,9 +742,7 @@ static u64 timer_read(struct clocksource *cs) */ if (!irqs_disabled() && !in_interrupt() && !in_softirq() && !time_travel_ext_waiting) - time_travel_update_time(time_travel_time + - TIMER_MULTIPLIER, - false); + time_travel_forward_time(TIMER_MULTIPLIER, false); return time_travel_time / TIMER_MULTIPLIER; } =20 --- base-commit: 58720809f52779dc0f08e53e54b014209d13eebb change-id: 20231020-uml-time-backwards-552c81aedd23 Best regards, --=20 Vincent Whitchurch