What kind of functions cannot be used in the critical section of the spin lock?

Question

I'm kind of confused about what sort of functions are not allowed in a spin lock's critical section.

In particular I'm confused about reentrant functions. I thought that all reentrant functions are unsafe to use in a spin lock's critical section, but it appears that functions like kfree and memcpy are ok to use.

So how do we know what functions are ok or not ok? I generally think anything that might block is unsafe, but don't all reentrant functions have the capacity/potential to block?

Also what is the role and relationship of the interrupt handler to spin locks?

@MarcoBonelli So is the only restriction on what you can do inside a spin lock is to not sleep? — Shisui, Oct 28 '21 at 23:09
Yeah, that's basically it. You must be careful not to call any function that has some possibility to sleep. — Marco Bonelli, Oct 28 '21 at 23:13

qz- · Answer 1 · 2021-10-29T03:29:16.543

0

You have reentrant backward: Re-entrant functions are the functions that are safe to use while a spinlock is held.

Many non-reentrant functions can be ok as well.

The primary concern is calling a function that directly or indirectly tries to acquire the same lock you're currently holding which leads to deadlock.

Specific to kmalloc, though you did not specify this in your question:

The general answer above is the reason why you've heard kmalloc may not be safe to use within a spinlock: It also acquires a spinlock by default. Use the flag GFP_ATOMIC to control whether kmalloc attempts to acquire a spinlock:

GFP_ATOMIC The allocation is high priority and must not sleep. This is the flag to use in interrupt handlers, in bottom halves, while holding a spinlock, and in other situations where you cannot sleep.

http://books.gigatux.nl/mirror/kerneldevelopment/0672327201/ch11lev1sec4.html

edited Oct 29 '21 at 03:29

answered Oct 28 '21 at 23:56

qz-

674
1
4
14

Malloc is reentrant and it's not safe to use while a spin lock is being held, so I don't think this is true – Shisui Oct 29 '21 at 00:39
@Shisui Where did you head that from? – qz- Oct 29 '21 at 00:51
Do you mean `kmalloc`? Is your question focused on the Linux kernel or not? `kmalloc` has a `get_free_pages`-style bitmask argument which determines what sort of reentrancy requirements it meets. E.g. it is more reentrant with `GFP_ATOMIC`. – Kaz Oct 29 '21 at 00:52
@Kaz yes, kmalloc. – Shisui Oct 29 '21 at 02:31
@Shisui I've updated the answer with information relevant to your `kmalloc` concerns. – qz- Oct 29 '21 at 03:29

What kind of functions cannot be used in the critical section of the spin lock?

1 Answers1