On Tue, Jun 6, 2017 at 9:54 PM, Lv Zheng <lv.zheng@xxxxxxxxx> wrote: > Considering this case: > 1. A program opens a sysfs table file 65535 times, it can increase > validation_count and first increment cause the table to be mapped: > validation_count = 65535 > 2. AML execution causes "Load" to be executed on the same table, this time > it cannot increase validation_count, so validation_count remains: > validation_count = 65535 > 3. The program closes sysfs table file 65535 times, it can decrease > validation_count and the last decrement cause the table to be unmapped: > validation_count = 0 > 4. AML code still accessing the loaded table, kernel crash can be observed. > > This is because orginally ACPICA doesn't support unmapping tables during > OS late stage. So the current code only allows unmapping tables during OS > early stage, and for late stage, no acpi_put_table() clones should be > invoked, especially cases that can trigger frequent invocations of > acpi_get_table()/acpi_put_table() are forbidden: > 1. sysfs table accesses > 2. dynamic Load/Unload opcode executions > 3. acpi_load_table() > 4. etc. > Such frequent acpi_put_table() balance changes have to be done altogether. > > This philosophy is not convenient for Linux driver writers. Since the API > is just there, developers will start to use acpi_put_table() during late > stage. So we need to consider a better mechanism to allow them to safely > invoke acpi_put_table(). > > This patch provides such a mechanism by adding a validation_count > threashold. When it is reached, the validation_count can no longer be > incremented/decremented to invalidate the table descriptor (means > preventing table unmappings) so that acpi_put_table() balance changes can be > done independently to each others. > > Note: code added in acpi_tb_put_table() is actually a no-op but changes the > warning message into a warning once message. Lv Zheng. > This still seems to be unnecessary gymnastics to keep the validation count around and make it work for random drivers. Which ACPI tables might be hot removed? If it's only a small handful of tables why not teach the code that handles those exceptional cases to manage a dedicated reference count mechanism? That way the other cases can be left alone and not worry about balancing their references. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
