x86/sgx: Silence softlockup detection when releasing large enclaves

Vijay reported that the "unclobbered_vdso_oversubscribed" selftest
triggers the softlockup detector.

Actual SGX systems have 128GB of enclave memory or more.  The
"unclobbered_vdso_oversubscribed" selftest creates one enclave which
consumes all of the enclave memory on the system. Tearing down such a
large enclave takes around a minute, most of it in the loop where
the EREMOVE instruction is applied to each individual 4k enclave page.

Spending one minute in a loop triggers the softlockup detector.

Add a cond_resched() to give other tasks a chance to run and placate
the softlockup detector.

Cc: [email protected]
Fixes: 1728ab5 ("x86/sgx: Add a page reclaimer")
Reported-by: Vijay Dhanraj <[email protected]>
Signed-off-by: Reinette Chatre <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Jarkko Sakkinen <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Tested-by: Jarkko Sakkinen <[email protected]>  (kselftest as sanity check)
Link: https://lkml.kernel.org/r/ced01cac1e75f900251b0a4ae1150aa8ebd295ec.1644345232.git.reinette.chatre@intel.com

Author: rchatre

arch/x86/kernel/cpu/sgx/encl.c

@@ -410,6 +410,8 @@ void sgx_encl_release(struct kref *ref)
 		}
 
 		kfree(entry);
+		/* Invoke scheduler to prevent soft lockups. */
+		cond_resched();
 	}
 
 	xa_destroy(&encl->page_array);