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Wed, 12 Jun 2024 01:56:14 -0700 (PDT) Message-ID: <92584a2d-6695-4884-ba2e-990842318d8a@suse.com> Date: Wed, 12 Jun 2024 10:56:13 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/7] x86/smp: do not use shorthand IPI destinations in CPU hot{,un}plug contexts To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: Andrew Cooper , George Dunlap , Julien Grall , Stefano Stabellini , xen-devel@lists.xenproject.org References: <20240610142043.11924-1-roger.pau@citrix.com> <20240610142043.11924-2-roger.pau@citrix.com> <615582c8-c153-424d-bce4-eb0c903d28ad@suse.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 12.06.2024 10:09, Roger Pau Monné wrote: > On Tue, Jun 11, 2024 at 09:42:39AM +0200, Jan Beulich wrote: >> On 10.06.2024 16:20, Roger Pau Monne wrote: >>> Due to the current rwlock logic, if the CPU calling get_cpu_maps() does so from >>> a cpu_hotplug_{begin,done}() region the function will still return success, >>> because a CPU taking the rwlock in read mode after having taken it in write >>> mode is allowed. Such behavior however defeats the purpose of get_cpu_maps(), >> >> I'm not happy to see you still have this claim here. The behavior may (appear >> to) defeat the purpose here, but as expressed previously I don't view that as >> being a general pattern. > > Right. What about replacing the paragraph with: > > "Due to the current rwlock logic, if the CPU calling get_cpu_maps() does so from > a cpu_hotplug_{begin,done}() region the function will still return success, > because a CPU taking the rwlock in read mode after having taken it in write > mode is allowed. Such corner case makes using get_cpu_maps() alone > not enough to prevent using the shorthand in CPU hotplug regions." Thanks. > I think the rest is of the commit message is not controversial. Indeed. >>> --- a/xen/arch/x86/smp.c >>> +++ b/xen/arch/x86/smp.c >>> @@ -88,7 +88,7 @@ void send_IPI_mask(const cpumask_t *mask, int vector) >>> * the system have been accounted for. >>> */ >>> if ( system_state > SYS_STATE_smp_boot && >>> - !unaccounted_cpus && !disabled_cpus && >>> + !unaccounted_cpus && !disabled_cpus && !cpu_in_hotplug_context() && >>> /* NB: get_cpu_maps lock requires enabled interrupts. */ >>> local_irq_is_enabled() && (cpus_locked = get_cpu_maps()) && >>> (park_offline_cpus || >> >> Along with changing the condition you also want to update the comment to >> reflect the code adjustment. > > I've assumed the wording in the commet that says: "no CPU hotplug or > unplug operations are taking place" would already cover the addition > of the !cpu_in_hotplug_context() check. Hmm, yes, you're right. Just needs a release-ack then to go in (with the description adjustment). Jan